Abstract

The Bouguer anomaly map of the Lac des Iles area constitutes a number of distinct gravity highs and lows that are associated, respectively, with the mafic-ultramafic and granitic components of a late Archean plutonic terrane. The apparent density and vertical derivative maps were computed to assist in the identification and the delineation of lithologies. From the apparent density map, several distinct density zones are recognized within the mafic-ultramafic intrusions. The identification of the density zones and their apparent densities are in good agreement with the mapped lithological groups and measured densities.

Gravity models in 2½-D were computed along six selected profiles. The Mafic-ultramafic Lac des Iles Complex with a mean density of 3.13 g/cm3 is characterized by a 30-mgal gravity high. The complex is modeled as a flattened funnel-shaped, steeply inward-dipping, intrusive body, part of which extends to a depth of about 4.5 km. The Tib Gabbro, represented by a 16-mgal gravity high, is modeled as a tilted, steeply inward-dipping, basin-like intrusive body extending to a depth of 3 km.

Gravity maxima for most of the mafic intrusions are located in proximity to faults in tonalite gneiss country rocks. These faults may have acted as conduits, allowing the emplacement of discrete pulses of mantle-derived magma. Gravity models show that mafic magma locally underplated granitic plutons and that mafic plutons typically have limited depth extents relative to their horizontal dimensions. These models indicate that intraplating by mafic magmas contributed to crustal thickening in the late Archean.

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